1. Field of the Invention
The present invention relates to an air sterilization device with low aerosol bounce. Particularly, it relates to an air sterilization device that contains a porous filtration media with a coating layer and a UV (ultraviolet) light generator. So, the bounce effect of aerosols can be significantly reduced. The maintenance cost is low. The germicidal effective rate is excellent. Plus, the flow rate of the air conditioning system remains high.
2. Description of the Prior Art
The conventional air sterilization methods at least include: using a filter to capture the solid aerosols in the air, utilizing UV light to emit on bacteria in the air directly; coating with a photocatalysis layer to cooperate with a UV light source to activate the photo-catalytic reaction in order to kill the bacteria, etc.
The first conventional method is to use a filter to capture the solid aerosols in the air. Referring to FIGS. 1 and 2, the conventional air conditioning system has a pipeline 90 (or ventilation duct). A general filter 80 is installed in a proper position (such as near the fresh air entrance or near exit) of this pipeline 90. This general filter 80 has a thickness X. The general filter 80 consists of many filtration fibers 81. There are many small openings 82 among the filtration fibers 81 for allowing air to flow through. The porosity of a general filter 80 is large; therefore the flow resistance (or called pressure drop) is small. The flow resistance and price of the filtration fibers 81 are significantly lower than the ones of foam or the ones of a high efficiency particulate air filter (or briefly referred as HEPA filter). As shown in FIG. 3, the eighteenth curve L18 represents a general filter 80. The nineteenth curve L19 represents foam. The twentieth curve L20 represents the HEPA filter. Theoretically, dusts or particles in the airstream can be captured by the filter fibers 81 based on the single fiber theory. The single fiber theory includes gravitational settling, inertial impaction, interception, diffusion, and electrostatic attraction. Because the porosity of the general filter is too large and the packing density of the filter is too small, the filtration efficiency cannot be increased. As a result, the collecting efficiency of the general filter is low. It is also lower than the one of the foam or the HEPA. Referring to FIG. 4, concerning the penetration rate of aerosols, the eighteenth curve L18 represents a general filter 80. The nineteenth curve L19 represents foam. The twentieth curve L20 represents the HEPA filter. Therefore, in order to achieve the high filtration efficiency, low flow resistance is required, because the flow resistance will significantly influence the energy consumption. Moreover, a good filter must have an excellent collecting efficiency and low flow resistance. That is, it must have a satisfactory filter quality (briefly referred as qF). The filter quality can be defined by the following equations (1) and (2).
                              q          F                =                                            -              ln                        ⁢                                                  ⁢            P                                Δ            ⁢                                                  ⁢            p                                              (        1        )            P=1−Eff  (2)
where                P: aerosol penetration;        Δp: flow resistance (or pressure drop);        Eff: collection efficiency.        
The flow resistance is the overall loss for the air flow through this filter, and is proportional to the filter thickness, packing density, flow velocity and the total surface area of the filter fibers. Hence, if the collection efficiency increases, the flow resistance also increases. By reducing filter flow velocity and adding more filter material in the same filter volume, the filter collection efficiency will improve. However, it increases the total cost for filter material. It is possible to happen that some of the solid aerosols 83 (or particles) with greater inertial force impact on the filter fibers 81 and then bounce off, as illustrated in FIG. 5 (the effect of bounce off). It will make some aerosols 83 to occur the re-entrainment phenomenon. Therefore, many aerosols 83 continue to penetrate through the filter material. It significantly reduces the collecting efficiency of the filter. For most air condition system (including heating, ventilating and air conditioning, briefly called HVAC), the flow velocity in the pipeline is high as in the range of 50˜300 cm/s (or even higher) depending on the capacity and application scope of the air conditioning system. Thus, once the flow velocity increases, the flow resistance increases, too.
The second conventional method is to utilize UV light to emit on bacteria in the air directly. The best wavelength of UV light is 253.7 nm (in the range of Ultraviolet C, short wave; briefly called UV-C; 280 nm-100 nm). However, the flow velocity in the pipeline is roughly 50˜300 cm/s. The aerosols carried by the airstream are also moving at that velocity. Under such high flow velocity, the time of UV light exposure is too short to kill these biological aerosols.
The third conventional method is to coat with a photocatalysis layer to cooperate with a UV light source to activate the photo-catalytic reaction to kill the bacteria. The photocatalysis layer can decompose some biological aerosols (or bacteria) into CO2 and water. Titanium dioxide (TiO2) is a commonly used photocatalysis. While being exposed to UV light or the solar light, it generates free radicals (hydroxyl radicals: OH) and creates electron-hole pairs, so that it can oxidize an organic object. The energy level is 3.2 eV for the anatase form of the titanium dioxide in a photochemical reaction. When titanium dioxide is exposed by the light having the wavelength less than 385 nm, electron will be exerted to the conduction band and leave one electron-hole that reacts with neighboring H2O and OH−. Therefore, in order to achieve the sterilization effect, the photocatalysis layer must contact with the target (such as the biological aerosols). In this sterilization system, the biological aerosols must contact with those hydroxyl radicals to achieve the function of sterilization.
However, if the filter material is coated with a photocatalysis layer, it does not consider that such system might work due to the bounce effect. Especially, the bounce effect is obvious for those aerosols with larger size. Under this condition, the overall collection efficiency will decrease. If someone wants to coat with a coating layer (to reduce the effect of aerosol bounce off) on the photocatalysis layer, the photocatalytic reaction will not work well due to low contacting portion between biological aerosols and photocatalysis layer. Besides, the photocatalysis layer is quite expensive. If this system is installed in a pipeline of an air conditioning system, it will create many problems. Furthermore, usually UV-A (Ultraviolet A, long wave, or called UVA; 400 nm-315 nm) is applied to activate the photocatalytic reaction. However, the UV-A has a less efficiency to kill the biological aerosols directly. The major function of UV-A is to activate the photocatalytic reaction.
The problems of these conventional methods can be listed below.
[1] It is hard to capture the bioaerosols (a brief term for biological aerosols) with larger size due to the bounce effect. For those bioaerosols with large size, they have high moving velocity due to inertial force. Once they impact on a surface, they tend to bounce off. So, the overall collection efficiency is low. According a study (Aino, N, 1993), the average diameter of virus is about 0.02˜0.3 μm. The average diameter of fungal spore or bacteria is about 3˜100 μm. For example, small aerosols (such as virus) can be captured by diffusion or electrostatic attraction. Large aerosols (such as fungal spore, bacteria, dust, etc.) can be captured by inertial impaction, interception, and gravitational settling. About the inertial impaction, the objects might be bounce off, so that they still can penetrate through the filter. Particularly, the fungal spore and bacteria can bounce off and spread away via the central air conditioning system in a hospital. It is hard to image how terrible the consequence is.
[2] HEPA is expensive and has high flow resistance. When a HEPA filter is used, the collection efficiency can increase to 99.97% or higher. It seems to be an ideal solution to capture bioaerosols (including the bacteria, fungal spore, pathogens, etc.). It also can avoid the bounce effect. However, the maintenance cost of HEPA filter is too high. Filter in any air conditioning system needs to be replaced periodically. The price of the HEPA filter is approximately ten times higher than the one of a general filter. Besides, because the collection efficiency is high, the collected particles or aerosols will stay in the filter to block the air and then form a blocking portion (or called a dust cake). As a result, the flow resistance is increased without any limit. In order to avoid that, usually the replacement period will be shortened. For example, the maintenance period should be shortened from once a month to twice a month. That means the cost is double per month. In addition, when the flow resistance increases to a certain level, the fan of this air conditioning system will consume more electricity. For example, if someone uses the HEPA filter to replace the general filter, the maintenance at least increases 20 times (10×2). The cost is extremely high.
[3] The sterilization effect is poor. In a traditional air condition system, there is no any sterilization equipment. Although sterilization equipment is installed, it might be installed on at the entrance or the exit of the system. There is no sterilization equipment installed in the middle of the pipeline. For those bioaerosols captured by the filter, they will not die immediately. These bioaerosols are just be stuck at a place. In case of the environment is good for growing up (for example: having enough water (high relative humidity) and nutrition), these stuck bioaerosols still can remain survive, multiply and even reproduce. Hence, that filter becomes the home of bioaerosols. The reproduced bioaerosols also can be widely spread out to more rooms and corners via the pipeline of the air conditioning system. More people might be infected. If the filter is coated with a photocatalysis layer, it does not consider overcoming the bounce effect. Therefore, the collection efficiency for large aerosols is low. The photocatalysis layer is useless, if the filter is covered by another material to reduce the bounce effect, the photocatalysis layer becomes invalid. Besides, the UV-A is used to activate the photocatalyic reaction. The sterilization effect of UV-A is relative lower than UV-C. In addition, the photocatalysis layer must contact with the bioaerosols. If the non-biological aerosols are stuck on some portion of the photocatalysis layer, only the rest portion of the photocatalysis layer still can conduct the photocatalytic reaction. Thus, the overall sterilization effect becomes low. Generally, the non-biological aerosols are more than the biological aerosols in the air. So, the function of photocatalysis layer will be reduced by the non-biological aerosols.
In addition, in the pipeline, all the aerosols are carried by the airstream under the flow velocity about 50˜300 cm/s. If one system only utilizes the UV light as the germicidal irradiation, the flow retention time seems too short to let the UV light work well. Furthermore, the pipeline includes many branches and sub-branches. Usually there is no any filtration device or sterilization equipment to capture these aerosols. Besides, aeorosols are possible to enter the pipeline, branches or other sub-branches via some connecting gaps, broken holes, cracks, water leaking, and so on. The aerosols can spread out through the pipeline. For example, there are patients, doctors, and nurses in the hospital all the time. It is impossible to vacate the entire hospital to conduct a full-hospital sterilization. Thus, the aerosols stayed in the pipeline might spread out and make all the members (including critical patients, patients in respiratory therapy, hospital workers, etc.) in the hospital to be infected. It is a very serious problem.